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Author(s):Ross, Sederra D.
Contributor(s):Reilly, Neil J.; Hewett, Daniel M.; Flores, Jonathan
Abstract:Resonance-stabilized radical (RSR) isomers of C$_9$H$_9$ persist in complex energetic environments such as flames and plasmas derived from aromatic precursors. Of the myriad possible C$_9$H$_9$ RSRs, only 1-indanyl (the global minimum) and 1-phenylallyl have been conclusively identified spectroscopically. Electronic spectra near 600\,nm, close to several DIBs, were recently reported by the group of Maier for C$_9$H$_9$ products of a heptadiyne discharge. They were tentatively ascribed to isomers of vinylbenzyl on the basis of computed transition energies and ionization potentials, the best agreement with the latter property (\textit{ca.} 7.3\,eV) provided by the meta isomer. To further examine this conjecture, we have undertaken surveys for the electronic spectra of \textit{o}-,\textit{m}-, and \textit{p}-vinylbenzyl radicals in discharges of vinyltoluenes, using resonant two-color ionization and fluorescence spectroscopy. In a jet-cooled discharge of $m$-vinyltoluene, we have detected $cis$- and $trans$-$m$-vinylbenzyl radicals near 525\,nm. We observe adiabatic ionization energies (\textit{ca.} 7.15\,eV) for both conformers that are comfortably bracketed by B3LYP (7.11\,eV) and CBS-QB3 (7.22\,eV) calculations, from which we conclude that the carrier of the 600\,nm spectrum remains unidentified. Optical-optical holeburning spectroscopy has been used to untangle $cis$ and $trans$ features of $m$-vinylbenzyl. There is very little to distinguish the two conformers from calculations, thermochemically or spectroscopically: they are similarly stable, their ground and excited state equilibrium geometries are planar, and their electronic transition energies, AIEs, predicted rotational contours, and excited state vibrational frequencies are highly similar. The most significant point of difference is a large (for $cis$) and relatively small (for $trans$) increase in the vinyl torsion force constant upon excitation, strongly suggesting identification of the $cis$ conformer from an origin dispersed fluorescence spectrum, acceptance of which identification allows several other ground-state assignments to fall into place. A considerable breakdown in mirror symmetry between excitation and emission spectra for $a'$ modes is tentatively attributed to interference between Franck-Condon and Herzberg-Teller contributions to the transition moment, the low symmetry of the molecule ($C$$_s$) placing little restriction on such an interaction.
Issue Date:23-Jun-20
Publisher:International Symposium on Molecular Spectroscopy
Citation Info:APS
Date Available in IDEALS:2020-06-26

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